Bidirectional electrowetting actuation with voltage polarity dependence

ABSTRACT

A novel electrowetting system for the smooth continuous movement of a droplet across a single circuit using a continuous applied voltage regardless of polarity. The actuation of the droplet is achieved by introducing a diode into the idealized electrical circuit of the electrowetting system. The diode is in parallel with a capacitor (dielectric) and effectively shorts the droplet on the side of a lower potential electrode so that the entire voltage drop is across the dielectric over the opposite electrode. This creates an energy imbalance that moves the droplet towards the higher potential. If the voltage polarity is reversed, the direction of actuation will reverse as well.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation in part of prior filed InternationalApplication, Serial Number PCT/US2010/060,763 filed Dec. 16, 2010, whichclaims priority to U.S. provisional patent application No. 61/286,944,entitled “BIDIRECTIONAL ELECTROWETTING ACTUATION WITH VOLTAGE POLARITYDEPENDENCE,” filed on Dec. 16, 2009, the contents of which are herebyincorporated by reference.

GOVERNMENT SUPPORT

This invention was made with government support under Grant Nos.CMMI-0600266 and CMMI-0927637 awarded by the National ScienceFoundation. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electrowetting system. More specifically,it relates to an electrowetting system for the smooth continuousmovement of a droplet across a single circuit using a continuous applieddirect current voltage.

2. Description of the Prior Art

Recent technical advances have enabled the manipulation of small volumesof fluids—often in discrete droplets. Many of these systems utilize thephenomena of electrowetting to manipulate the small electrical droplets.Electrowetting on dielectric (EWOD) is the reduction in apparent contactangle of a fluid droplet using the capacitance of a dielectric layerwhich lies between the droplet and an electrode. Applying voltage acrossthis barrier (between the droplet and electrode) causes charge migrationto occur within the droplet and electrode, which modifies the apparentsurface energy of the droplet, causing its apparent contact angle to bereduced.

Applications for electrowetting, in general, are diverse, ranging fromthe shaping of microlenses, fiber optics switching, display technology,and optical filters, to such interesting areas as the creation of smalllow-power-consumption motors. When looking at applications for EWODdroplet transport, the most significant (with much interest anddiversity of research) is lab-on-a-chip designs. Also, the use ofdroplet motion to assist in assembly of nano- and micro-scale componentsfor microdevices holds promise.

Droplet movement by asymmetric electrowetting (where only a portion ofthe droplet has its contact angle reduced causing droplet motion) isknown in the prior art. The prior art uses successive activation ofdiscrete small electrodes, several of which are covered by the droplet.As each electrode is activated and the contact angle above it reduced,the droplet is “handed off” from electrode to electrode. Thisaccomplishes droplet movement in successive discrete steps each having amagnitude equivalent to the size of the electrodes and requires complexcontrol systems to control the activation of electrodes in the propersequence and with the proper timing. Moreover, because electrowettingbehavior is related to the voltage squared, it is typically shows nosignificant dependence on voltage polarity.

Accordingly, what is needed is an electrowetting system for the smoothcontinuous movement of a droplet across a single circuit using acontinuous voltage.

What is also needed is an electrowetting system that is dependent on thepolarity of the applied voltage, i.e., the actuation direction changeswith a change in voltage polarity.

However, in view of the prior art considered as a whole at the time thepresent invention was made, it was not obvious to those of ordinaryskill in the art how the limitations of the art could be overcome.

SUMMARY OF INVENTION

The claimed invention is a novel electrowetting system for the smoothcontinuous movement of a droplet across a single circuit using acontinuous applied voltage regardless of polarity. The actuation of thedroplet is achieved by introducing a diode into the idealized electricalcircuit of the electrowetting system. The diode is in parallel with acapacitor (dielectric) and effectively shorts the droplet on the side ofa lower potential electrode so that the entire voltage drop is acrossthe dielectric over the opposite electrode. This creates an energyimbalance that moves the droplet towards the higher potential. If thevoltage polarity is reversed, the direction of actuation will reverse aswell.

Generally speaking, the electrowetting system includes: (1) an electrodelayer adapted to act as a resistor; (2) a dielectric layer adapted toact as a capacitor disposed in overlying relation to the electrodelayer; (3) a plurality of diodes adapted to act as diodes in parallelwith the dielectric layer and disposed within the dielectric layer; (4)a hydrophobic surface treatment layer adapted to act as a capacitor inparallel with the dielectric layer and the plurality of diodes anddisposed in overlying relation to the electrowetting system; (5) anelectrolyte droplet disposed in overly relation to the hydrophobicsurface treatment layer; and (6) a voltage applied to the electrowettingsystem, causing said electrolyte droplet to move due to a potentialdifference in the electrowetting system.

In an embodiment, the electrowetting system uses the current-rectifyingproperties of oxide films of the so-called valve metals to construct thediodes. By patterning an array of holes in the dielectric layer, underwhich lies a valve metal electrode, and applying a potential across theelectrode, the side of the droplet above the anodic hole will have itscontact angle reduced. As the droplet flows to cover thenext-most-positively charged hole, the previously anodic hole willbecome cathodic (allowing current flow) and the newly-covered hole willbecome anodic, causing the contact angle to be reduced over it. Thissequence is repeated until the potential is removed or the array ofholes ends.

In an embodiment, electrochemical diodes are created by holes in thedielectric that expose an aluminum electrode to NaCl solution. Thealuminum electrode self-passivates and prevents current flow in onedirection. This creates a diode-like electrical behavior. Other salt andacidic solutions including Na2SO4, tartaric acid and citric acid couldbe used.

In an embodiment, the electrowetting system is fabricated by: (1)providing a silicon substrate; (2) applying a layer of SiO₂ in overlyrelation to said silicon substrate; (3) patterning a layer ofphotoresist in overlying relation to said layer of (4) etching voids insaid layer of SiO₂; (5) applying a layer of aluminum in overly relationto said phototresist and within said etched voids in said layer of SiO₂;(6) removing said layer of photoresist along with unwanted aluminum; and(7) coating said eletrowetting system with a layer of hydrophobicsurface treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1 depicts the natural (or Young's) contact angle when no voltage isapplied in the prior art;

FIG. 2 depicts the reduced contact angle (and resulting dropletasymmetry) when voltage is applied only to the right-hand electrode inthe prior art;

FIG. 3 depicts the idealized equivalent circuit of the prior art EWODdesigns for droplet transport;

FIG. 4 depicts idealized equivalent circuit of the claimed inventionincludes a diode;

FIG. 5 depicts the idealized equivalent circuit of an array ofdiode-capacitor sites demonstrating how continuous droplet motion isachieved; and

FIG. 6 illustrates the method of fabricating the electrowetting system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As collectively depicted in prior art FIGS. 1 and 2, electrowettingresults in a reduction of contact angle in the presence of an electricalpotential difference between the droplet 12 and the electrode 14 it liesupon. To reduce the contact angle of only a portion of the droplet 12(to achieve droplet motion/transport), there must exist a difference inpotential only between a portion of the drop and substrate (or thepotential difference between droplet and substrate must be significantlygreater in one portion of the droplet compared to another). In order toachieve asymmetry in electrical potential, the substrate must bemanipulated. As discussed above in the prior art, the paradigm has beento use a series of small electrodes 14, several of which are covered bythe drop 12 at any one time. In this way, only the portion of dropletimmediately above an energized electrode 14 has its contact anglereduced, and motion can be achieved by sequentially energizing thedesired electrodes no that the leading edge of the droplet repeatedlyexperiences contact angle reduction, moving forward a small amount ineach step.

A simplified electrical schematic of the prior art devices might looksomething like FIG. 3. The circuit consists of the droplet 12, a voltagesource 16, a switch 18 to control which of the series of electrodes 14is energized, and a dielectric layer 20. The dielectric layer 20 ismodeled as idealized electrical components, namely capacitors 40. If anadditional composite dielectric layer is used (a dielectric layer toppedby a surface treatment) there would be a second capacitor placed inseries with the one representing the dielectric layer. In this design,the electrode 14 used is highly conductive and as such is modeled to beat equipotential with the applied voltage source 16. The design (andmany other electrowetting designs) necessitates grounding of the circuitthrough the top of the droplet 12. For cases of static electrowetting(where the droplet 12 does not move) electrical contact can be made assimply as placing a wire into the bulk of the droplet 12. In cases ofelectrowetting where the droplet moves, a common technique has been touse a single continuous top plate electrode (not shown) so that thedroplet 12 is always grounded so long as its top remains in contact withthe top plate electrode.

Still referring to FIG. 3, only one electrode 14 is energized. As such,there is contact angle reduction only on the right side of the droplet12, resulting in a small motion to the right. Once the droplet has movedto cover the next electrode 14 to the right, it will be in turnenergized, and the previously energized electrode 14 switched off. Thisresults in another small motion to the right. In this way, an electricalpotential difference is always maintained at the right hand (leading)edge of the droplet 12.

As depicted in FIG. 4, the electrowetting system of the claimedinvention achieves droplet motion by creating a potential differencebetween only a portion of the droplet 12 and substrate by theintroduction of a diode 22. Diode 22 permits droplet 12 movement using amuch simpler design, and allows for continuous motion (rather than insmall discrete steps as in the prior art). Moreover, the introduction ofa diode 22 into the system makes the system dependent upon the polarityof the voltage. The entire device consists of a single circuit.

As depicted in FIG. 5, a voltage is applied across a substrate electrode14 atop which are patterned a series of diodes 22 and capacitors 40(dielectric layer) in parallel. Any two adjacent diodes 22 will be at adifferent potential (due to the resistance of the electrode 14), thedifference being a function of the total voltage applied across theelectrode and the geometry of the device (namely the total length ofelectrode and spacing between adjacent diodes). When two adjacent diodes22 are connected electrically by the presence of the droplet 12, themore positively charged diode 22 will be reverse-biased (preventing theflow of current) and the more negatively charged diode 22 will beforward-biased (allowing current flow). The result is a potentialdifference between the droplet 12 and electrode 14 at the reverse-biaseddiode 22 but not at the more negatively charged forward-biased diode 22.This potential difference combined with the capacitance 40 provided bythe dielectric layer results in a contact angle reduction throughelectrowetting effects in the area surrounding the reverse-biased diode22, while the equipotential between the droplet 12 and forward-biaseddiode 22 means the voltage in the Young-Lippmann equation is essentiallyzero resulting in no contact angle reduction in the area surrounding theforward-biased diode 22.

To achieve motion, the prior art required the operator to switch to thenext electrode in sequence to achieve another step of motion, and so onuntil the total motion was complete. In contrast, in the electrowettingsystem of the claimed invention, as depicted in FIG. 5, this is notnecessary. The resistance of the electrode 14 between diode sitesprovides for a voltage drop between them, resulting in each diode beingat a different potential than any other. As the droplet moves to theright and covers the next-most positively charged diode, that diode willbecome reverse-biased, preventing current flow and resulting in apotential difference between the droplet and surrounding electrode.Meanwhile, the previously reverse-biased diode is now more negativelycharged than the newly covered diode, and as such becomes forwardbiased, allowing current flow and eliminating the contact anglereduction. The net effect of all this is that the droplet motion willcontinue automatically across the length of the electrode in thedirection of positive voltage gradient until either the pattern ofdiodes ends or the voltage is removed, requiring no switching ofindividual electrodes. Additionally, the polarity of the applied voltagedetermines the direction of droplet motion, whereas the prior artelectrowetting setups show little to no dependence of response onvoltage polarity. Another significant difference between our design andthe prior art is the lack of need for a top plate electrode. In theclaimed invention, the voltage is applied across the electrode (notbetween droplet and electrode) and the droplet only provides a parallelcurrent path as it covers the various diodes. As such, the need for atop plate electrode vanishes, allowing more leeway in designing devices.

Example Embodiment #1

In an embodiment, the electrowetting system includes at least fourcomponents, each corresponding to one of the idealized electricalcomponents discussed previously and depicted in FIG. 5. The systemincludes a doped silicon electrode substrate and is the equivalent tothe resistors shown in FIG. 5. The next component, deposited atop theelectrode layer, is the dielectric layer. This layer provides themajority of the capacitance needed to achieve the electrowetting effect.The next components are the valve metal sites. These provide theselective current rectification behavior needed for the electrowettingsystem to function, and as such can be equated to the diodes discussedpreviously. The final component includes a thin surface treatment layer.While the surface treatment will have an effect on the total capacitanceof the system (and so may be considered as a portion of the capacitor)its major impact is not as an electrical component. In addition toacting as a portion of a composite dielectric layer, it providessignificant mechanical benefits to the electrowetting system byproviding an extremely non-wetting interface between droplet andsubstrate. Alternatively, a hydrophobic material may be used as thedielectric layer.

In the electrowetting system, the voltage term used in theYoung-Lippmann equation to model contact angle reduction is not thetotal voltage applied across the entire electrode, but only thepotential difference between the two adjacent valve metal sites. This ismerely a function of the total applied voltage and the spacing betweensites. In order to keep this value high enough to result in asignificant difference in contact angle from one side of the droplet toanother (and hence cause motion), it is necessary to use voltages muchhigher than is typical of most prior art EWOD designs where the voltagebetween electrode and electrolyte is the total applied voltage. To keepthe current through the electrode and electrolyte at a minimum, a highresistivity substrate electrode is used. For availability andcompatibility with microfabrication techniques, a large doped siliconwafer is used as a substrate in one embodiment, and once fabrication iscompleted, individual test strips are diced from it.

Because the electrode is a silicon wafer substrate, the simplestdielectric layer to use is a thermally-grown silicon dioxide (SiO₂)layer. This provides a robust dielectric layer that is easily grown,patterned and etched with standard microfabrication techniques, andprovides satisfactory performance according to our model.

In order for the necessary selective current flow to occur at regularintervals, there must be periodically spaced portions of the electrodewhere the dielectric layer has been removed. In the design presentedhere, this is achieved by patterning small circular holes in thedielectric layer. Alternative designs could include strips of removeddielectric oriented perpendicular to the direction of desired dropletmotion.

The spacing of the holes in the dielectric layer is a key designparameter. It must be such that for a given diameter droplet, at leasttwo sites are always covered by the droplet. If at any time a dropletcovered only one site, the equivalent circuit would not include parallelpaths through both the substrate and electrolyte droplet (as currentwould only flow through the substrate) and droplet motion would cease.If on the other hand the hole spacing is made very very small, thevoltage drop between any two adjacent sites would be such that theeffective voltage across the drop would not be sufficient to ensuresignificant contact angle reduction at the leading edge.

The final piece in a functional device is the use of a valve metal layerat the bottom of the holes through the dielectric layer, separating theexposed electrode and the droplet itself. If this layer were notpresent, current could flow indiscriminately between electrode anddroplet at all hole sites. The diode-like current rectificationproperties of valve metals allows for current to flow only when thevalve metal is more negatively charged than the electrolyte. Since muchof our previous work in exploring this diode-like behavior was doneusing thin aluminum films, it seemed reasonable to continue with thismaterial. Aluminum is a valve metal which exhibits robust diode-likeproperties and is easy to deposit in pure layers of controlledthicknesses.

Metals commonly considered valve metals include tantalum, niobium,aluminum, zirconium, hafnium, tungsten, bismuth and antimony. Otherelements, namely beryllium, magnesium, silicon, germanium, tin,titanium, and uranium, exhibit some of these properties and aresometimes counted in the ranks of valve metals. Successful actuation hasbeen demonstrated with both aluminum and silicon sites. The use ofsilicon is favorable because it eliminates the need for depositing metalin the dielectric holes if a silicon substrate is used.

The final layer is a surface treatment applied to the entire top surfaceof the finished electrowetting system. The addition of a hydrophobicsurface treatment aids in strong and dependable electrowettinq behavior.By increasing the natural contact angle of the three phase contact line,it allows for contact angle reductions of greater magnitude. Inaddition, it has been shown that a highly hydrophobic layer also reducesthe severity of hysteresis. The Young-Lippmann equation indicates thatthe contact angle reduction for a given voltage should be identical,regardless of whether the angle is advancing or retreating (voltage isincreasing or decreasing).

The hydrophobic surface treatment used in this design reduceshysteresis, allowing for more dependable performance. Also, when theintent of an EWOD design is to achieve droplet transport, hydrophobiclayers tend to reduce the occurrence of ‘pinning,’ where a portion ofthe droplet will stick to the substrate. This pinning can result ineither the cessation of droplet motion, or if the actuation force actingon the drop is sufficient it may cause a portion of the drop to continueits motion while the remainder remains anchored to the area of wherepinning occurs causing the droplet to break in two.

To reduce the chances of pinning of the droplet at the valve metalsites, the design presented here covers the entire surface of the waferwith a hydrophobic layer. On first examination it would seem that theaddition of a dielectric layer atop the valve metal would reduce oreliminate the necessary current flow at the negatively charged (trailingedge) site. However, the surface treatment layer applied is very thinand has a relatively low dielectric constant. Enough current leakageoccurs across this layer that it does not significantly reduceperformance, due to its natural porosity, and defects or otherelectrically conductive pathways.

A key benefit of the hydrophobic layer is its effect on the naturalcontact angle of the droplet-surface interface. This helps in achievingdroplet motion in two related ways. First, by starting off with a lardernatural contact angle, the total possible change in contact angle isgreater. Secondly, since the actuation force acting on the droplet isthe result of the asymmetry in contact angle from leading to trailingedge, the greatest actuation force is achieved by having the greatestpossible natural contact angle and reducing it to the lowest valuepossible on the leading edge.

As mentioned above, the additional thickness provided by the hydrophobiclayer and its dielectric properties has the effect of decreasing thetotal capacitance between electrode and droplet, which has an impact onelectrowetting performance. The magnitude of this impact is determinedby the hydrophobic layer used (and its dielectric constant) and thethickness of the applied layer.

CYTOP™ 809M (Asahi Glass is a useful surface treatment. Thisfluoropolymer provides a large natural contact angle and is easily laiddown in thin consistent-thickness coats.

Example Embodiment #2

EWOD substrates with aluminum electrodes and 2.1 μm thick CYTOP™dielectric layers were prepared. Defects in the dielectric layer wereintroduced by scratching the samples with a probe tip on amicropositioner. A 50 μl droplet of 1 mM NaCl solution was placed overthe scratched area and voltage was applied to a probe placed in thedroplet while the aluminum electrode was grounded. Aluminumself-passivates and prevents current flow in one direction. This createsa diode-like electrical behavior with diode breakdown voltage over 100V.

Example Embodiment #3

The eletrowetting system is manufactured using microfabricationtechniques, including thermal oxidation, photolithographic patterningand etching, electron-beam vapor deposition, and spin-coating. Thesubstrate used for all iterations produced was a 4″ diameter 270 μmthick wafer of silicon N-doped to a resistivity 300-500 Ω-cm. Using apredictive model (and assuming test strip widths of 8.98 mm) this wouldresult in Joule heating of 0.37 to 0.62 W/cm² depending on where in therange of given resistivities the wafer is. Atop this wafer an oxidelayer was grown using dry thermal oxidation to a final oxide thicknessof 480 nm.

Initially several wafers were oxidized simultaneously to the same finalthickness, with those not ready for immediate further processing setaside for later use. Later in the testing process it was decided that itwould desirable to fabricate wafers with a slightly thinner oxide layer.Before further processing of these wafers, they were immersed in astandard hydrofluoric acid buffered oxide etch (BOE) to reduce totalthickness to approximate 350 nm. Once the wafers held the desiredthickness of oxide, they were prepared for photolithography.

This begins by first spin-coating a primer of Hexamethyldisilazane(HMDS) to ensure a good bond between oxide and photoresist. Afterpriming, a layer of Shipley S1813 positive photoresist was applied viaspin-coat. After spin-coating the wafer was subjected to a soft bake ona hotplate at 100° C. It was then masked and exposed, then submersed inMicroposit MF-319 developer to remove the portions of photoresist thatwere exposed to UV during the exposure step. After a hard bake for oneminute at 100° C. the now-exposed portions of oxide were etched awaydown to bare silicon by BOE.

After the photolithographic process is completed, but before removal ofthe unexposed photoresist, the entire wafer was coated with a layer ofaluminum using electron-beam physical vapor deposition. Final thicknessof the aluminum layer was 300 nm. This step results in aluminum coveringthe entire wafer, while the desired result is to have aluminum only atthe bottom of the holes etched in the dielectric layer duringphotolithography (as well as small strips of aluminum deposited directlyto the substrate electrode on each end, to aid in making electricalcontacts). Since the photoresist layer remains from previous steps)covering all those areas which still have an oxide layer, when it isremoved by rinsing in acetone it takes with it the aluminum that wasdeposited atop it. After this liftoff procedure, the result is a siliconwafer with an oxide layer atop it. In this oxide layer are a series ofpatterned holes, at the bottom of which (deposited directly atop thebare silicon) each contains a layer of aluminum.

All that remains is the surface treatment layer. The initial test waferdesign called for a final CYTOP thickness of 180 nm. This was achievedusing spin-coating. For each step, once the final spin speed is reachedit is maintained until the total step duration has been reached

Example Embodiment #4

It may be possible to create or improve diode-like characteristics byapplying particular electrolytes. Diode-like behavior may be achievedbased solely on the choice of electrolyte. For example, Citric Acid andTartaric Acid are known to improve passivation of Aluminum.

Example Embodiment #5

We have demonstrated an additional embodiment in which the aluminum isnot deposited in the holes so that silicon acts as the diode material.

It will thus be seen that the objects set forth above, and those madeapparent from the foregoing disclosure, are efficiently attained. Sincecertain changes may be made in the above construction without departingfrom the scope of the invention, it is intended that all matterscontained in the foregoing disclosure or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindisclosed, and all statements of the scope of the invention that, as amatter of language, might be said to fall there between.

GLOSSARY OF CLAIM TERMS

Electrowetting: modification of the ability of a liquid to maintaincontact with a solid surface with an applied electric field

Dielectric: an electrical insulator that can be polarized by an appliedelectric field.

Diodes: a two-terminal electronic component with an asymmetric transfercharacteristic.

Hydrophobic: the physical property of a molecule that is repelled from amass of water.

Doping: intentionally introducing impurities into an extremely puresemiconductor for the purpose of modulating electrical properties.

Wafer: thin slice of semiconductor material.

Photoresist: light sensitive material.

Fluoropolymer: a fluorocarbon based polymer with strong carbon-fluorinebonds. Characterized by a high resistance to solvents, acids, and bases.

1. An electrowetting system for the smooth continuous movement of adroplet regardless of voltage polarity, comprising: an electrode layeradapted to act as a resistor in said electrowetting system; a dielectriclayer disposed in overlying relation to said electrode layer, saiddielectric later adapted to act as a capacitor in said electrowettingsystem; and a plurality of diodes disposed within said dielectric layer,said plurality of diodes adapted to act as diodes in parallel with saiddielectric layer.
 2. An electrowetting system for the smooth continuousmovement of a droplet regardless of voltage polarity as in claim 1,further comprising: a hydrophobic surface treatment layer disposed inoverlying relation to said electrowetting system, said hydrophobicsurface treatment layer adapted to act as a capacitor in parallel withsaid dielectric layer and said plurality of diodes.
 3. An electrowettingsystem for the smooth continuous movement of a droplet regardless ofvoltage polarity as in claim 2, wherein: an electrolyte droplet disposedin overly relation to said hydrophobic surface treatment layer.
 4. Anelectrowetting system for the smooth continuous movement of a dropletregardless of voltage polarity as in claim 3, wherein: said plurality ofplurality of diodes are disposed in said dielectric and spaced apartsuch that at least two of said diodes are covered by said droplet as anygiven time.
 5. An electrowetting system for the smooth continuousmovement of a droplet regardless of voltage polarity as in claim 1,wherein: said electrode substrate is a doped silicon wafer.
 6. Anelectrowetting system for the smooth continuous movement of a dropletregardless of voltage polarity as in claim 1, wherein: said dielectriclayer is silicon dioxide.
 7. An electrowetting system for the smoothcontinuous movement of a droplet regardless of voltage polarity as inclaim 1, wherein: said plurality of diodes are a valve metal.
 8. Anelectrowetting system for the smooth continuous movement of a dropletregardless of voltage polarity as in claim 1, wherein: said plurality ofdiodes are Al exposed to an aqueous solution comprising compoundsselected from the group consisting of NaCl, NaSO₄, Citric Acid, orTartaric acid.
 9. An electrowetting system for the smooth continuousmovement of a droplet regardless of voltage polarity as in claim 2,wherein: the hydrophobic surface treatment layer is a fluoropolymer. 10.An electrowetting system for the smooth continuous movement of a dropletregardless of voltage polarity, comprising: an electrode layer acting asa resistor in said electrowetting system; a dielectric layer disposed inoverlying relation to said electrode layer, said dielectric later actingas a capacitor in said electrowetting system; a plurality of diodesdisposed within said dielectric layer, said plurality of diodes actingas diodes in parallel with said dielectric layer; a hydrophobic surfacetreatment layer disposed in overlying relation to said electrowettingsystem, said hydrophobic surface treatment layer acting as a capacitorin parallel with said dielectric layer and said plurality of diodes; anelectrolyte droplet disposed in overly relation to said hydrophobicsurface treatment layer; and responsive to a voltage applied to saidelectrowetting system, said electrolyte droplet moves due to a potentialdifference in said electrowetting system.
 11. A method of fabricating anelectrowetting system for the smooth continuous movement of a dropletregardless of voltage polarity, comprising the steps of: providing asilicon substrate; applying a layer of SiO₂ in overly relation to saidsilicon substrate; patterning a layer of photoresist in overlyingrelation to said layer of SiO₂; etching voids in said layer of SiO₂;applying a layer of aluminum in overly relation to said phototresist andwithin said etched voids in said layer of Sio₂, said aluminum layerbeing in contact with said silicon substrate; removing said layer ofphotoresist along with unwanted aluminum; and coating saidelectrowetting system with a layer of hydrophobic surface treatment.